A raster output scanner

Abstract: A pulsed imaging Raster Output Scanner (80) utilizes pulse width modulation in conjunction with asymmetric spatial filtering (136) to prevent overshoot of the intensity and maintain precise intensity levels into the other exposure levels. In one embodiment tri-level exposures are created.

Description: The present invention relates generally to a pulsed imaging, non-facet tracked, pulse width modulation Raster Output Scan (ROS) system for creating tri-level exposures at a recording medium such as images at a photosensitive surface, and, more specifically, for asymmetric spatial filtering in the ROS system. In tri-level, highlight color imaging, unlike conventional xerography, upon exposure, three charge levels are produced on the charge-retentive surface. The highly charged (i.e. unexposed) areas are developed with toner of one color, and the area more fully discharged is also developed, but with a toner of a different color. Thus, the charge retentive surface contains three exposure levels; zero exposure, intermediate exposure, and full exposure, which correspond to three charge levels. These three levels can be developed to print, for example, black, white, and a single color. There are several scanning techniques known in the prior art to obtain the tri-level exposure imaging. A conventional flying spot scanner uses a ROS unit to "write" an exposed image on a photoreceptive surface a pixel at a time. To obtain higher spatial resolution, a pulse imaging scanner can be utilized. A preferred technique, capable of higher spatial resolution is to use similar optical elements as the flying spot scanner (rotating polygon, laser light source, pre polygon and post polygon optics), but with an A/O modulator which illuminates many pixels at a given time, resulting in a scanner with a coherent imaging response. With this type of scan system, the exposure level, or levels at the image surface, can be controlled by controlling the drive level of the A/O modulator dependent on the video data. In a tri-level system, two drive levels are used, one for the white exposure and a second higher drive level for the DAD or color exposure. One such system is described in EP-A-0 533 346. EP-A-0 533 346 discloses a raster output scanner in which pulse width modulation and spatial filtering are used to form three exposure levels at the surface of a recording medium. The scanner includes a light source, an acousto-optic modulator, a polygon scanner, and Fourier transform means. The modulator modulates a high intensity beam generated by the light source in response to pulse width modulated signals. The scanner has a scanning element having a plurality of facets and scans the modulated beam across the recording medium. The Fourier transform means transforms the beam and spatial filters it to limit the transmission to predetermined frequencies. In previous Scophony scanners the polygon facet itself functions as the spatial filter. "Scophony spatial light modulator", Johnson et al., Optical Engineering, Jan/Feb 1985, Vol. 24, No. 1, pages 93 to 100, discloses a scanner architecture which utilises the spatial modulation capability of an acousto-optic modulator. The scanner includes a laser light source, an acousto-optic modulator and a rotating polygon mirror to scan the laser beam from the light source across an image plane. A pre-polygon objective lens performs a Fourier transform on the laser beam profile and projects it onto the polygon facet which acts as a spatial bandpass filter to block out some spatial frequency components and transmitting others. A post-polygon objective lens performs an inverse Fourier transform of the spatially filtered spectrum and projects this beam onto the image plane. A vestigial sideband scanner arrangement is also disclosed in which the optical alignment is shifted so that the dc spatial frequency falls just inside one edge of the polygon facet to improve the scanner response. However, a tri-level xerographic system is extremely power sensitive so the spatial filter within the raster output scanner must be more sensitive than merely the polygon facet. The polygon facet also present problems with start of scan and end of scan spatial filtering while the polygon facet is spinning. A stationary spatial filter is preferable. The polygon facet as spatial filter does not provide a variable, adjustable filter. The polygon facet as spatial filter also does not permit adjusting of the frequency band that is to be reflected. The polygon facet as spatial filter does not correct for flare. Overshoot, in the context of a tri-level xerographic system, is the unwanted power intensity above the intermediate level (all on white). In a highlight color tri-level xerographic system, black is the off level, white is the intermediate level, and color is the highest level. Overshoot can cause print defects. When the overshoot or unwanted excess power intensity is severe enough, the prints acquire unintended color toner to appear on the page. In a pulsed image system, the contrast of the pixel produced by the raster output scanner (ROS) is directly related to the overshoot. In a pulsed image ROS, contrast is controlled by the spatial filter. The spatial filter controls the contrast by allowing certain spatial frequencies to pass while blocking off some undesired frequencies. It is an object of this invention to provide a non-polygon facet spatial filter in a pulsed imaging, pulse width modulation Raster Output Scanner for creating tri-level exposures to control overshoot and maintain precise intensity levels. In accordance with one aspect of the present invention, there is provided a pulsed imaging, non-facet tracked, pulse width modulation raster output scanner for creating tri-level exposures on a recording medium comprising: a radiant energy source for providing a coherent focused beam, control circuit means for converting an image bit map video data stream into a composite analog video image data stream, an acousto-optic modulator for modulating said beam in response to the analog image video data stream simultaneously applied thereto to provide a modulated optical output, optical means for performing a Fourier transformation of the modulated optical output, and a rotatable scanning element interposed between the recording medium and the radiant energy source, the scanning element having a plurality of facets for intercepting the filtered optical output and repeatedly scanning the output across the surface of the recording medium; characterised in that the scanner further comprises an asymmetric spatial filtering means for limiting predetermined frequencies of the fourier transformed modulated optical output to produce a filtered optical output which forms the tri-level exposures without overshoot. Instead of obtaining an intermediate exposure level by controlling the acoustic amplitude, an intermediate exposure is provided by using pulse width modulation in a pulse imaging system in conjunction with spatial filtering. Use of a pulsed imaging scanner with pulse width modulation, however, may result in image quality problems. A separate asymmetric filter in the pre-polygon optics corrects for overshoot of the power intensity in the ROS. The present invention will be described further, by way of examples, with reference to the accompanying drawings, in which:- Figure 1 is a schematic view of the pulsed imaging, pulse width modulation raster output scanner for tri-level exposure, Figure 2A is a schematic view is the video pulse sequence for one bit on/one bit off for a full width pulse, Figure 2B is a schematic view is the video pulse sequence for one bit on/one bit off for a partial width pulse, Figure 3A is a schematic view of the intensity profile of the fourier transform of the full width pulse of Figure 2A, Figure 3B is a schematic view of the intensity profile of the fourier transform of the partial width pulse of Figure 2B, Figure 4A is a schematic view of the intensity profile of the full width pulse of Figure 3A after being filtered through a spatial filter, Figure 4B is a schematic view of the intensity profile of the partial width pulse of Figure 3B after being filtered through a spatial filter, Figure 5 is a schematic view of a white pulse width signal sequence, the corresponding fourier transform, the effect of a symmetric spatial filter and the resulting filtered exposure, Figure 6 is a schematic view of a white pulse width signal sequence, the corresponding fourier transform, the effect of an asymmetric spatial filter and the resulting filtered exposure, Figure 7 is a schematic view of a one-white, one-black pulse width signal sequence, the corresponding fourier transform, the effect of a symmetric spatial filter and the resulting filtered exposure, and Figure 8 is a schematic view of a one-white, one-black pulse width signal sequence, the corresponding fourier transform, the effect of an asymmetric spatial filter and the resulting filtered exposure. Reference is now made to Figure 1 wherein there is disclosed a non-facet tracking, pulsed imaging raster output scanner 80 with pulse width modulation for tri-level exposure for a highlight color imaging system. The raster output scanner 80 uses a Scophony scanner architecture. A light source 82 provides the original beam of light 84 for the scanner 80. The light source 82 is preferably a laser, such as a helium-cadmium laser or a helium-neon laser, which generates a collimated beam of monochromatic light 84. The monochromatic light beam is focused by a beam expander lens system 86 of a cylindrical lens 88 and a spherical lens 90 onto modulator 92. The modulator 92 is an acousto-optic Bragg cell or, as it is more commonly called, an acousto-optic modulator. The acousto-optic modulator 92 is used to modulate the light beam 84 in accordance with the information contained in the electrical video signal supplied to the modulator 92 by control circuit 94. The beam 84 is modulated by the individual bits of the pulse sequences of the video signal from the modulator control circuit 94. The control circuit 94 converts an image bitmap video data stream into an analog video data stream consisting of a plurality of pixel periods. The simplest and fastest video pulse sequence for the video data stream is one bit on/one bit off as shown in Figure 2A. The pulse 96 takes up the full width 98 of the pulse sequence to form a full pulse width. A pulse 100 in Figure 2B (still in the one bit on/one bit off pattern) which does not fill the full width 98 of the pulse sequence is a partial pulse width or pulse width modulated. The partial pulse can be any percent of the full width 98 of the pulse sequence from 0% to 100%, although 15% to 70% is preferable, and 50% is illustrated in Figure 2B. When pulse width modulation is being applied to reduce the exposure level of a string of "on" pixels, the average light level that passes through the modulator is proportional to the duty cycle of the pulse stream. However, since the sideband energy does not pass beyond the facet, the average light level is further reduced, the actual reduction depending on the higher harmonic content that is passed to the acoustic wave. In practice, it can be expected that pulse width modulation at a 50% duty cycle will result in an exposure level of about 25%. The video bit pulse sequence is translated into individual acoustic pulses in sequence in the modulator 92 which in turn modulates the light beam 84. The collimated beam of light 84 is modulated by modulator 92. Without a video signal applied to the modulator 92 by control circuit 94, only a non-diffracted, zero order output beam 102 is produced. The non-diffracted beam 102 is absorbed by a beam stop 104 for printing a uniform zero exposure along the scan line of the recording medium. Typically, application of a video signal to the modulator 92 by the control circuit 94 will produce two significant output beams: a first-order, modulated beam 106 and the zero-order, non-diffracted, non-modulated beam 102 which is absorbed by the beam stop 104. The modulated beam 106 has a spatial profile defined by the video bit signal pulse and a spatial intensity which is a function of the amplitude of the video signal applied to the modulator. The pulse width narrowed video signals which have been pulse width modulated become spatially narrow optical pulses. The modulated beam 106 from the modulator 92 is recollimated by an anamorphic lens system 108. The anamorphic lens system 108 consists of a spherical lens 110, a cylindrical lens 112 and a spherical lens 114 The anamorphic lens system 108 also performs a fourier transform on the fast scan plane portion of the modulated beam 106 to form a fourier transformed, modulated beam 116. The full pulse width beam of Figure 2A after recollimation and fourier transformation by the anamorphic lens system will produce an intensity profile 118 in the Fourier plane as shown in Figure 3A. The intensity profile 118 has a single central lobe 120 and two side lobes 122. The spacing 124 between a side lobe 122 and the central lobe 120 is described by the equation (f 0 W EFL) where f 0 is the spatial frequency of the video pattern, W is the wavelength of light and EFL is the effective focal length of the anamorphic lens system 108. The partial pulse width beam or pulse width modulated beam of Figure 2B after recollimation and fourier transformation by the anamorphic lens system will produce a intensity profile 126 in Figure 3B similar to the intensity profile 118 in Figure 3A. The partial pulse 100 of Figure 2B is only a fractional width of the full pulse 96 of Figure 3A. The intensity profile 126 in Figure 3B also has a single central lobe 128 and two side lobes 130. However, the spacing 132 in Figure 3B between a side lobe 130 and the central lobe 138, using the equation (f 0 W EFL), is twice as large as the spacing 124 in Figure 3A. The spatial frequency of the partial pulse 100 is twice that of the full pulse 96 in the one bit on/one bit off pulse pattern. As previously stated, the partial pulse 100 is 50% of the full pulse 96, so that the frequency of the one bit on/one bit off pattern is twice. The fourier transformed, modulated beam 16 is then filtered by a spatial frequency bandpass filter 136. The spatial filter 136, which can be a simple slot filter, will block the farther spaced apart side lobes 130 of the intensity profile 126 of the partial pulse width modulated beam as shown in Figure 4B while allowing the closer spaced side lobes 122 of the full pulse width beam to pass through the filter 136 as shown in Figure 4A. The limited size of the slot acts as a Fourier plane spatial frequency bandpass filter that limits the upper frequency that is reflected to the photoreceptor. In Figure 4B, it is seen that the frequency associated with a 1-on/1-off pattern is passed through the optical system, thereby allowing printing of that frequency. Figure 4A shows the diffraction pattern for printing a uniform intermediate exposure. The pulse width modulated video pattern is turned on and off for each pixel, with the on time corresponding to the desired exposure level. The frequency of this pattern is twice that of the 1-on/1-off pattern and thus, the associated diffraction lobes are blocked by the spatial filter. This spatial frequency filtering of the optical signal results in a uniform intermediate level output. Thus, the intensity profile 118 of the full pulse width beam will be at full intensity from the central lobe and the two side lobes for printing a uniform full exposure along the scan line of the recording medium. The spatially filtered intensity profile 126 of the partial pulse width modulated beam will contain only the central lobe for printing at a lower, partial intensity the same pattern (one bit on/one bit off in this example) as the full width pulse beam. The spatially filtered intensity profile 126 of the partial pulse width modulated beam will print a uniform intermediate exposure along the scan line of the recording medium. The intermediate exposure level is obtained from the pulse width narrowed video signals which become spatially narrow optical pulses exiting the modulator and are filtered by the spatial filter to result in a low uniform exposure at the recording medium. The spatial filter 136 is stationary in the optical path since the raster output scanner 80 is non-facet tracking and the beam 116 is not moving to track the rotating polygon mirror facet. The modulated, collimated, fourier-transformed, filtered beam 138 from the spatial filter 136 is reflected from the facet 140 of the rotating polygon mirror 142 through a scan angle from the rotation of the polygon facets. The reflected beam from the rotating polygon mirror 142 passes through a f-theta lens system 144 of a negative cylindrical lens 146 and a positive cylindrical lens 148 for focusing the beam in the fast scan plane. The beam then passes through a wobble correction system 150 consisting of a cylindrical lens 152 and a cylindrical mirror 154 for correcting for wobble in the slow scan plane. The f-theta lens system 144 and wobble correction system 150 focus the resulting beam 156 at a point 158 along a scan line 160 along the surface of a recording medium 162 which is sensitive to the spatial intensity profile of the beam. As shown, medium 162 is preferably a xerographic drum which is rotated in the direction of arrow 164 to provide the Y direction of scan. The recording medium can be the photosensitive medium of a photoreceptor. As noted previously, the fourier transform by the anamorphic lens system 108 and the subsequent filtering by the spatial filter 136 results in a uniform exposure of the spatial intensity of the beam 156 as either full exposure, intermediate exposure or zero exposure at the recording medium 162. The intermediate exposure level is obtained from the pulse width narrowed video signals which become spatially narrow optical pulses exiting the modulator 92 and are filtered by spatial filter 136 to result in a low uniform exposure at the recording medium 162. The zero exposure level is obtained from a non-modulated beam which exits the modulator and is blocked by the beam stop. In this configuration, an image of an acousto-optic video stream from the acousto-optic modulator is imaged to the recording medium. The imaging optics are composed of a set of anamorphic pre-polygon optics and f-theta post-polygon scan optics. The rotating polygon is placed at the back focal plane of the set of scan optics and at the front focal plane of the pre-polygon optics. The rotating polygon causes the optical image of the acousto-optic video pattern to sweep across the scan line of the recording medium with a certain velocity in addition to the image velocity. Acoustic image motion at the surface of the recording medium which, if uncorrected, would blur the optical image, is canceled by balancing the acoustic and scan velocities with the pre-polygon and post-polygon optics magnification, resulting in the acoustic image remaining stationary on the recording medium. A white pulse width modulated signal sequence 166 in Figure 5 has a fourier transform, modulated intensity profile 168 with a single central lobe 170 and two side lobes 172 and 174. The symmetric spatial frequency bandpass filter 176 will block the two side lobes 172 and 174 of the intensity profile 168 while allowing the central lobe 170 to pass therethrough. The central lobe 170 of the fourier transform, modulated intensity profile 168 will produce an intermediate or white exposure 178 in the tri-level xerographic system. A one-white, one-black pulse width modulated signal sequence 180 in Figure 6 has a fourier transform, modulated intensity profile 182 with a single central lobe 184, two closely spaced side lobes 186 and 188 and two farther spaced side lobes 190 and 192. The symmetric spatial frequency bandpass filter 176 will block the two farther spaced side lobes 190 and 192 of the intensity profile 182 while allowing the central lobe 184 and the two closely spaced side lobes 186 and 188 to pass therethrough. The central lobe 184 and the two closely spaced side lobes 186 and 188 of the fourier transform, modulated intensity profile 182 will produce a sinusoidal exposure 194 centered as an intermediate or white exposure but with overshoot 196 into the full exposure or color level exposure in the tri-level xerographic system. An asymmetric spatial frequency bandpass filter 198 of Figures 7 and 8 reduces the overshoot in the tri-level xerographic system. A white pulse width modulated signal sequence 200 in Figure 7 has a fourier transform, modulated intensity profile 202 with a single central lobe 204 and two side lobes 206 and 208 The intensity profile is not centered symmetrically within the spatial frequency bandpass filter 198. The central lobe 204 of the intensity profile 202 is not centered within the spatial frequency bandpass filter 198. Rather, the spatial frequency bandpass filter 198 is asymmetrical with respect to the fourier transform, modulated intensity profile 202. However, the asymmetric spatial frequency bandpass filter 198 will block the two side lobes 206 and 208 of the intensity profile 202 while allowing the central lobe 204 to pass therethrough. The central lobe 204 of the fourier transform, modulated intensity profile 202 will produce an intermediate or white exposure 210 in the tri-level xerographic system. A one-white, one-black pulse width modulated signal sequence 212 in Figure 8 has a fourier transform, modulated intensity profile 214 with a single central lobe 216, two closely spaced side lobes 218 and 220 and two farther spaced side lobes 222 and 224. The intensity profile 214 is not centered symmetrically within the spatial frequency bandpass filter 198. The central lobe 216 of the intensity profile 214 is not centered within the spatial frequency bandpass filter 198 Rather, the spatial frequency bandpass filter 198 is asymmetrical with respect to the fourier transform, modulated intensity profile 214. The asymmetric spatial frequency bandpass filter 198 will block the two farther spaced side lobes 222 and 224 and one of the closely spaced side lobes 218 of the intensity profile 214 while allowing the central lobe 216 and one of the closely spaced side lobes 220 to pass therethrough. The central lobe 216 and the one closely spaced side lobe 220 of the fourier transform, modulated intensity profile 214 will produce a sinusoidal exposure 226 centered as an intermediate or white exposure but without overshoot into the full exposure or color level exposure in the tri-level xerographic system. The asymmetric frequency bandpass spatial filter will greatly reduce the overshoot above the intermediate or white level of the tri-level xerographic system. This reduction in overshoot is achieved by reducing the overall energy transmitted by removing one of the side lobes without eliminating the necessary modulation frequency of the signal. The spatial filter 136 of Figure 1 can be a liquid crystal spatial frequency bandpass filter. The spatial filter 136 can be positioned anywhere along the optical path between the anamorphic lens system 108 and the f-theta lens system 144. The key factor is the beam to be filtered must be fourier transformed and collimated in the fast scan plane. The beam after the reflection from the polygon facet is moving through a scan angle. A spatial filter positioned between the polygon mirror and the f-theta lens system, therefore, would have to move in synchronization with the beam. A slot filter could be physically moved or the aperture image could be moved in a stationary liquid crystal filter. The one bit on/one bit off pulse pattern is used as an illustrative example. Other bit pulse patterns from the video drive signal can be used to form pulsed imaging. Other bit pulse patterns will have different spatial intensity profiles after fourier transformation but the profiles will still consist of a pattern of lobes. Pulse width modulation to form partial width pulses will have side lobes father spaced apart after fourier transform. Some of these spaced lobes will be blocked by the spatial filter yielding partial intensity or intermediate exposure along the scan line of the recording medium. The exact bit pulse pattern effects only the pattern of the exposure. The exact bit pulse pattern does not effect the uniformity of the exposure level nor whether it is a full exposure level or an intermediate exposure level. Since the modulator is not facet-tracking in a non-facet tracking scanner architecture, there is no modulator roll off of the imaging beam. If the modulator carrier frequency is kept constant at 105 MHz, a frequency which will not beat adversely with the modulated pulses, optical power fluctuations will be alleviated. The polygon size is increased to account for the non-facet tracking architecture to sufficiently to pass all relevant spatial information to the photoreceptor.

Claims: A pulsed imaging, non-facet tracked, pulse width modulation raster output scanner (80) for creating tri-level exposures on a recording medium (162) comprising: a radiant energy source (82) for providing a coherent focused beam, control circuit means (94) for converting an image bit map video data stream into a composite analog video image data stream, an acousto-optic modulator (92) for modulating the beam in response to the analog image video data stream simultaneously applied thereto to provide a modulated optical output (106), optical means (108, 110, 112, 114) for performing a Fourier transformation of the modulated optical output (106), and a rotatable scanning element (142) interposed between the recording medium (162) and the radiant energy source (82), the scanning element (142) having a plurality of facets (140) for intercepting the filtered optical output and repeatedly scanning the output across the surface of the recording medium (162); characterised in that the scanner further comprises an asymmetric spatial filtering means (198) for limiting predetermined frequencies of the fourier transformed modulated optical output to produce a filtered optical output which forms the tri-level exposures without overshoot. A scanner according to claim 1, wherein the asymmetric spatial filtering means (198) acts as a side band filter to those portions of the modulated beam corresponding to the pulse width modulated data signals to reduce the overall illumination intensity of the pulse width modulated data signals. A scanner according to claim 1 or 2, wherein the asymmetric spatial filtering means (198) comprises a slot filter. A scanner according to claim 1 or 2, wherein the asymmetric spatial filtering means (198) comprises a liquid crystal filter. A scanner according to any one of the preceding claims, further comprising a beam expanding lens system (86) consisting of a cylindrical lens (88) and a spherical lens (90) for focusing the beam onto the modulator (92). A scanner according to any one of the preceding claims, further comprising a f-theta lens system (144) consisting of two cylindrical lens (146, 148) for focusing the scanning beam in the fast scan plane across a line (160) on the recording medium (162). A scanner according to any one of the preceding claims, wherein the optical means (108) comprises an anamorphic lens system consisting of a spherical lens (110), a cylindrical lens (112) and a spherical lens (114) for recollimating and performing Fourier transformation of said modulated beam (106) in the fast scan plane. A scanner according to any one of the preceding claims, further including a wobble correction lens system (150) consisting a cylindrical lens (152) and a cylindrical mirror (154) for correcting for wobble in the slow scan plane. A scanner according to any one of the preceding claims, wherein the recording medium surface is exposed at three exposures levels: a zero exposure, a full exposure and, an intermediate exposure level.

Inventors: ANG ANTHONY, TAN CHYE S, ANG, ANTHONY, TAN, CHYE S
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1994
Main classification: H04N1
IPC codes: H04N1, G06K15, G02B27